The present invention relates generally to overload protection arrangements for electric motors and more particularly to line break protection systems for multispeed hermetic motor arrangements.
Thermal overload protection for motors is a highly developed art with a particularly inexpensive and readily available protective device being the line break motor protector. Such devices may take the form of a bimetallic disc or other element which normally connects two terminals together but which when overheated snaps to another position opening the circuit between those terminals. This bimetallic disc or other member may be self-resetting when it cools or may require a manual reset. The thermal protector may include a heating element in series between the two terminals so that the temperature responsive bimetallic member may actuate to open the circuit between the terminals in response to excessive current flow between those terminals so that the thermal protector is responsive to excessive current, excessive temperature of its environment or some combination of the two, any of which may be indicative of an overload condition. Such thermal protectors may include additional terminals and additional heating elements for certain installations.
Another highly developed technology is that of multiple speed motors. Multiple speed single-phase alternating current operation of an electric motor is frequently achieved by operating that motor in a selected one of several possible pole number configurations. This may be achieved by providing independent windings for the several different pole configurations or by providing windings which are differently interconnected to achieve the different pole configurations, with either option being available for either the auxiliary or starting winding, or the main or running winding.
When a given motor winding is active in more than one pole configuration, overload protection has heretofore taken the form of one or more temperature sensors in thermal contact with the motor windings and connected to an external solid state logic module, as illustrated, for example, in U.S. Pat. No. 3,978,382. Such systems are subject to a number of drawbacks. The cost of the sensors and logic module is significant. Separate and essentially independent arrangements are required for detecting excessive current and detecting winding temperature increase. System complexity increases the likelihood of system malfunction. Such systems fail to effectively utilize available high production and therefore low cost parts.